Fuel cell system and reverse flow relief valve

ABSTRACT

A method of shutting down operation of a fuel cell system comprising a fuel cell stack, the method comprising the sequential steps of: i) ceasing a supply of fuel to the fuel cell stack; ii) closing a shut-off valve on an exhaust line in fluid communication with a cathode system of the fuel cell system, the cathode system comprising a cathode fluid flow path passing through the fuel cell stack; iii) pressurising the cathode system with an air compressor in fluid communication with a cathode air inlet port in the fuel cell stack; and iv) ejecting water from the cathode flow path.

This application is a national stage filing under 35 U.S.C. 371 ofInternational Application PCT/GB2008/003256, filed on 25 Sep. 2008,(Published on 2 Apr. 2009 as WO/2009/040535), which claims priority fromGB Application No. 0718761.0, filed 26 Sep. 2007, the entirety of eachbeing incorporated herein by reference, as if fully set forth herein.

FIELD OF THE DISCLOSURE

The disclosure relates to the operation of, and apparatus relating to, afuel cell system, and in particular though not exclusively to a strategyfor shutting down a fuel cell system.

BACKGROUND

Water is integral to the operation of a fuel cell system, for example inthe form of the system described herein comprising a fuel cell stackbased around a proton exchange membrane (PEM). Reaction of protons(hydrogen ions) conducted through the PEM from an anode flow path, withoxygen present in a cathode flow path, produces water. Excess waterneeds to be removed from the fuel cell stack to avoid flooding andcausing a consequent deterioration in performance. An amount of water,however, needs to be present in at least the cathode flow path tomaintain hydration of the PEM, so as to achieve optimum performance ofthe fuel cell. Managing this water, by deliberate injection and removal,can also provide a useful mechanism for removing excess heat from thefuel cell stack.

To optimize performance, water can be employed deliberately in such fuelcell systems through injection into the cathode flow path of the stack.Such water injection fuel cell systems have potential advantages ofreduced size and complexity, as compared with other types of fuel cellsystems employing separate cooling channels. Water may be injecteddirectly into the cathode flow path through water distributionmanifolds, as for example described in GB2409763.

For water injection systems, it is important that any water fed backinto the cathode flow path is of high purity, so as to avoidcontamination of the PEM and consequent degradation of stackperformance. This requirement for high purity, however, means thatadditives to lower the freezing point of water cannot be used. Forautomotive applications in particular, typical requirements includestarting up from below freezing, typically as low as −20° C. toreplicate environments in which the fuel cell may be used in practice.Since high purity water has a freezing point of 0° C. (at 1 barpressure), any water left in the fuel cell system will, given sufficienttime, freeze after shut-down of the fuel cell.

Ice in the fuel cell system, and in particular within the cathode flowpath, can prevent the stack from operating properly, or even at all. Ifany part of the cathode flow path is blocked with ice, air cannot bepassed through the cathode and the fuel cell may not be capable ofself-heating to above freezing point. Other methods of heating the wholestack will then be necessary, which will require consumption of externalpower before the fuel cell can begin supplying electrical power and heatby itself.

A purging operation can be used on shut-down of a fuel cell stack, suchas that described in U.S. Pat. No. 6,479,177. This document discloses afuel cell stack having water cooling passages separate from the cathodeflow path. A pressurised dry nitrogen feed is used to purge water fromthe stack before allowing the temperature of the stack to fall belowfreezing. This method, however, requires a supply of pressurisednitrogen, which might not be available or even desirable in anautomotive environment.

SUMMARY

In a first aspect, the disclosure provides a method of shutting downoperation of a fuel cell system comprising a fuel cell stack, the methodcomprising the sequential steps of:

i) ceasing a supply of fuel to the fuel cell stack;

ii) closing a shut-off valve on an exhaust line in fluid communicationwith a cathode system of the fuel cell system, the cathode systemcomprising a cathode fluid flow path passing through the fuel cellstack;

iii) pressurising the cathode system with an air compressor in fluidcommunication with a cathode air inlet port in the fuel cell stack; and

iv) ejecting water from the cathode flow path.

In a second aspect, the disclosure provides a fuel cell systemcomprising:

a fuel cell stack;

a cathode system having a cathode fluid flow path comprising a cathodeair inlet line, a cathode volume within the fuel cell stack and acathode exit line connected in series and configured to allow passage ofair through the fuel cell stack;

an air compressor in fluid communication with the cathode air inletline;

a thermally insulated containment vessel configured to receive waterthrough a water return line from the cathode flow path,

wherein the fuel cell system is configured to eject water from thecathode flow path into the containment vessel through the water returnline upon shutting down operation of the system.

In a third aspect, the disclosure provides a reverse flow relief valvecomprising:

a first feed port;

a second feed port;

a non-return valve within a main fluid passage extending between thefirst and second feed ports, the non-return valve configured to allowfluid to pass from the first to the second feed ports and to blockpassage of fluid in the reverse direction;

a bypass fluid passage in fluid communication with the main fluidpassage;

a sealing valve biased against an end of the bypass passage between thebypass passage and a purge port,

wherein the sealing valve is configured to maintain a seal against thebypass passage when fluid pressure in the first feed port exceeds fluidpressure at the second feed port to prevent fluid flow from the mainfluid passage to the purge port through the bypass fluid passage, and toallow fluid flow from the second feed port to the purge port through thebypass fluid passage when fluid pressure at the second feed port exceedsfluid pressure at the first feed port.

In a fourth aspect, the disclosure provides a fuel cell systemcomprising:

a fuel cell stack;

a cathode system having a cathode fluid flow path comprising a cathodeair inlet line, a cathode volume within the fuel cell stack and acathode exit line connected in series and configured to allow passage ofair through the fuel cell stack;

a heat exchanger connected in series with a water separator to thecathode exit line of the cathode fluid flow path,

wherein a water ejection outlet line of the water separator is connectedto a water containment vessel by a first water return line.

DRAWINGS

The disclosure will now be described by way of example only, and withreference to the appended drawings in which:

FIG. 1 illustrates a schematic diagram of the arrangement of variouscomponents within an overall fuel cell system;

FIG. 2 illustrates a schematic diagram of an exemplary water containmentvessel;

FIG. 3 illustrates a cutaway perspective view of an exemplary reverseflow relief valve:

FIGS. 4 a and 4 b illustrate schematically the operation of the reverseflow relief valve of FIG. 3; and

FIGS. 5 a and 5 b illustrate schematically two alternativeconfigurations for cathode exit stream liquid separation.

Other features and advantages of the present disclosure will be setforth, in part, in the descriptions which follow and the accompanyingdrawings, wherein the implementations of the present disclosure aredescribed and shown, and in part, will become apparent to those skilledin the art upon examination of the following description taken inconjunction with the accompanying drawings or may be learned by practiceof the present disclosure.

ADDITIONAL DESCRIPTION

In the following description, various exemplary implementations, aspectsand characteristics are discussed as directed toward surgicalinstruments, tools, systems and methods more particularly applied to thespine. The focus on this application is not intended to be, nor shouldit act as, a limitation to the scope of this disclosure. The otherfeatures and advantages of the resent disclosure will be set forth inpart, in the descriptions which follow and the accompanying drawings,wherein the implementations of the present disclosure are described andshown, and in part, will become apparent to those skilled in the artupon examination of the following description taken in conjunction withthe accompanying drawings or may be learned by practice of the presentdisclosure.

Heading and Titles are not intended to be limitations and should be readin a general sense. Implementations may include several novel features,no single one of which is solely responsible for its desirableattributes or which is essential to practicing the disclosure describedherein. The advantages of the present disclosure may be attained bymeans of the instrumentalities and combinations particularly pointed outin the disclosure and any appended claims.

FIG. 1 shows a schematic diagram of an exemplary fuel cell system 100comprising a fuel cell stack 110 and other associated components. Thefuel cell stack 110 has a cathode flow path passing through it, thecathode flow path comprising an air inlet 124 leading to an air inletline 123 and into the stack at the cathode air inlet 126. After passingthrough an internal cathode volume (not shown) within the fuel cellstack 110, the cathode flow path exits the fuel cell stack 110 into thecathode exit line 121, through the cathode exhaust line 122 and anexhaust shut-off valve 120. During normal operation, the exhaustshut-off valve 120 is partially or fully open. Various components suchas a heat exchanger 130, with associated cooling fan 139, and a waterseparator 131 may be connected to or part of the cathode exit line 121and exhaust line 122 in the cathode flow path. Temperature sensors TX1,TX2, TX3, TX5 and pressure sensors PX2, PX3 may also be present,connected at appropriate places to monitor the inlet line 123 and exitline 121 of the cathode flow path.

The expression ‘cathode system’ in the present context is intended toencompass those parts of the fuel cell system 100 that are associatedwith the cathode volume within the fuel cell stack. These include thevarious internal components of the fuel cell such as the inlets,outlets, the internal flow path and water distribution structures, aswell as components in fluid communication with the cathode volume suchas the various inlet, outlet, recirculation and exhaust lines for bothliquids and gases. The term ‘cathode flow path’ is intended to encompassa subset of the cathode system that includes a fluid flow path from theair inlet 124 through an air compressor 133, the inlet line 123, thecathode volume of the fuel cell stack 110, and the cathode exit line121. The terms ‘anode system’ and ‘anode flow path’ are to beinterpreted similarly, with reference to the various components of thefuel cell system 100 associated with the anode volume.

The air compressor 133, connected to the cathode air inlet line 123,provides compressed air to the cathode flow path. Other components suchas an air inlet heat exchanger 134, a flow meter 135, one or more airfilters 136, 137 and an air heater 138 may be present in the cathodeinlet line 123 between the air inlet 124 and the fuel cell stack 110.The air inlet heat exchanger 134 may be used in conjunction with acoolant line 141, a three-way valve 142 and a temperature sensor TX7 topre-heat air from the air compressor 133 with coolant from the coolantline 141 during operation of the fuel cell system 100. The coolant line141 passing through the air inlet heat exchanger 134 forms a separatecooling circuit configured to extract heat from the air stream after thecompressor 133. This coolant line 141 is preferably operated after thefuel cell stack 110 reaches a normal operating temperature, in order toavoid extracting heat from the air inlet stream in the cathode air inletline 123 during start-up of the system 100. Diversion of coolant in theline 141 may be achieved through use of the valve 142, allowing controlover whether coolant is delivered to the heat exchanger 134. Since thecoolant line 141 is separate from water fed into the cathode system, therequirement for high purity water is not the same. The coolant used inthe coolant line 141 may therefore comprise additives such as glycol tolower the freezing point of the coolant used.

Fuel, typically in the form of gaseous hydrogen, enters the fuel cellsystem via a pressure-reducing valve 151 and an actuated valve 152,preferably in the form of a normally-closed solenoid-actuated valve. Thefuel supply 150, when in the form of hydrogen gas, is typically locatedremotely from the fuel cell system, for example in the form of apressurised tank towards the rear of a vehicle. A furthersolenoid-actuated valve 153 and a pressure-reducing valve 154 may beprovided closer to the fuel cell stack 110 in the fuel inlet line 155 ofthe anode flow path between the fuel source 150 and the anode inlet 156of the fuel cell stack 110. Two separate sets of valves are thereforeprovided leading to the anode inlet 156, one set 151, 152 near to thetank and the other set 153, 154 closer to the fuel cell stack 110, withan intermediate pressurised fuel line 119 in between. Thepressure-reducing valve 154 regulates the pressure of the dry fuel gasto a level suitable for introduction to the fuel cell stack 110. Thepressure-reducing valve 154 is preferably a passive device which has apreset pressure setting applied, although an actively controlled devicemay be used. A fuel heater 145 is optionally provided, for example inthe pressurised fuel line 119 before the valve 153, as shown in FIG. 1,or alternatively in the fuel inlet line 155 either before or after thepressure-reducing valve 154.

A further actuated valve 161 is provided on the anode exit line 165.Each actuated valve 152, 153, 161 may be provided with a local heaterelement to defrost the valve as required, although activation of thevalves 152, 153, 161 through passage of current through the solenoidwill provide a certain degree of heating. Preferably each of theactuated valves 152, 153, 161 is configured to be fail-safe, i.e. willonly open when actuated by current passing though the solenoid.

To monitor and to relieve pressure of fuel within the anode flow path, apressure sensor PX1 and/or pressure relief valve 157 may be provided.The pressure relief valve 157 is preferably set to open and exhaustfluid from the anode flow path through a pressure relief exhaust line158 when the pressure in the anode flow path exceeds a safe operatinglevel.

A further manually operable valve 162 in the anode exit line 165 may bepresent, this valve 162 being for used for example during servicing toensure depressurisation of the anode flow path. Water build-up in theanode flow path in the fuel cell stack 110 may occur, for example as aresult of diffusion of water through the PEM from the cathode side.Consequently, an anode exhaust water separator 163 may be provided inthe anode exhaust line 164 to separate any water present in the exhaustline 164. This water can be exhausted or optionally recirculated. Duringoperation of the fuel cell stack 110, the valve 161 is typically heldclosed, and only opened intermittently to exhaust any built-up waterfrom the anode fluid path.

A cathode water injection inlet 127 is provided in the fuel cell stack110, the inlet 127 connected to a cathode water injection line 125. Thecathode water injection line 125 may be heated along a part or the wholeof its length, and extends between a water containment vessel 140 andthe cathode water injection inlet 127. A heater 129 may be provided toapply heat to a specific region of the line 125 to heat water passingthrough the injection line 125 towards the cathode water injection inlet127. A further pressure sensor PX4 may be provided on the cathode waterinjection line 125 in order to monitor the back-pressure on the line 125during operation.

Water from the cathode exit line 121 is pumped with a water pump 132,optionally provided with a heater 143, through a water return line 128towards the water containment vessel 140, further details of which areprovided below with reference to FIG. 2. Excess water is ejected fromthe fuel cell system 100 out of the water containment vessel 140 througha water overflow line 144.

Shown in FIG. 2 is a schematic cross-sectional view of the watercontainment vessel 140 of FIG. 1. The vessel 140 comprises a thermallyinsulating wall 210 and a lid 211, which may also be thermallyinsulated. Preferably the wall 210 of the vessel 140 is of a double wallconstruction, having a vacuum or other thermally insulating layer suchas air or expanded polystyrene between the two walls. The inner surface215 of the vessel 210 is preferably made from a material having aresistance to corrosion, such as stainless steel, to preventcontamination of water 212 within the vessel.

The purpose of the lid 211 is to allow connections to the variouselements housed within the vessel 140, whilst also maintaining a gooddegree of insulation. Typically the lid 211 is manufactured fromglass-reinforced nylon with an additional layer of insulating foam.Ports in the lid to accommodate passage of the lines 125, 144, 128 arepreferably configured such that when the system is shutdown, anyresidual water runs back into the vessel. This involves using pipe of asuitably large diameter such that beads of water do not form to span theinternal bore of the pipe and hang up in the line. Preferably, nofittings are used in the lid so that pipes passing through the lid 211contain no sharp bends. In a general aspect, therefore, the waterinjection line 125 extending between the water containment vessel 140and the cathode water injection inlet 127, and an exhaust line 121, 128extending between a cathode exit line 121 and the water containmentvessel 140 both comprise piping having an internal bore such that beadsof water do not span the bore after ejection of water from the cathodesystem.

A thermostatic heating element 236 within the vessel 140 is provided tomaintain the temperature of water 212 within the vessel 140 abovefreezing point. A level sensor 233 provides a signal indicating thelevel of water 212 within the vessel. A heater 237 is provided inaddition to the thermostatic heating element 236 in order to providefaster heating to defrost the water 212 if frozen. Due to the energyrequirement of changing the phase of water from solid to liquid, thisheater 237 is typically of a higher power rating than the thermostaticheating element 236, for example around 180 W or higher. Thethermostatic heater 236 is configured to ensure that the temperature ofthe water 212 in the vessel 140 remains above a set point. This setpoint is typically 5° C., in order to prevent the water from freezing.The thermostatic heater 236 may be powered by a 12V battery supply, andset to operate for a prescribed period. Hence, during this period,liquid water in the vessel can be guaranteed. For longer periods atsub-zero ambient temperature, the thermostatic heater 236 is disabled tosave on battery power. The water 212 may then freeze, and will requiredefrosting with the higher power heater 237. The thermostatic heater istypically of a power rating such that a maximum heat output is slightlylarger than the maximum rated losses from the vessel. A typical powerrating is in the range of 2 to 4 W.

A temperature sensor TX4, preferably comprising a submerged thermistor,is installed in order to allow the temperature of the water 212 in thevessel 140 to be monitored.

An overflow line 144 is provided to eject excess water from thecontainment vessel if a level of water in the vessel exceeds a presetamount.

Water from the water return line 128 enters the vessel 140 through afilter 234. A pump 230, 231, 240 pumps the water 212 from the vessel 140through a further filter 214, a reverse flow relief valve 213 and intothe cathode water injection line 125. A flow meter 235 is configured tomonitor the amount of water passing through the cathode water injectionline 125.

The pump is preferably constructed such that a motor portion 231 islocated outside the containment volume of the vessel 140 and thereforeavoids being in contact directly with water 212 within the vessel. Ashaft 240 between the motor 231 and a pump head 230 allows the motor 231to drive the pump head 230. The pump head, comprising at least theinlet, outlet and impeller, is preferably of a construction such that,after being submerged in frozen water the pump is able to operate againonce the water is defrosted. The motor 231 is preferably rated foroperation at sub-zero temperatures.

The pump head 230 is located so as to be submerged by water 212 in thevessel 140. This has the advantage of there being no requirement for thepump head 230 to be purged during shutdown or heated during startup,particularly when water 212 is maintained within the flask aftershutdown. The pump head 230 is preferably configured to have a smallthermal mass. Thawing of any ice within the pump head 230 is achievedvia heat transferred from the surrounding water as it defrosts. The pumphead 230 is also preferably configured to accommodate expansion due toice formation. On thawing, the pump head 230 then returns to itsoriginal shape without compromising its operation.

The reverse flow relief valve 213 is constructed such that water isallowed to pass from the pump head 230 through the cathode waterinjection line 125 towards the cathode water injection inlet 127 whenthe pump is operational, creating a pressure drop across the valve inthe direction of flow. However, when the pump is stopped and pressure inthe cathode water injection line 125 is increased, the valve 213 allowswater to flow back into the vessel 140 through a purge port 238.

The purpose of the reverse flow relief valve 213 is to allow water to beback flushed into the vessel 140 from the fuel cell stack 110 andconnecting lines during shutdown of the system 100. Closing the cathodeexhaust valve 120 allows water in the fuel cell stack 110 to be forcedunder pressure from the air compressor 133 back through the cathodewater injection inlet 127, out of the stack 110 and through the waterinjection line 125 towards the water containment vessel 140. However, ifa gear pump is used in the containment vessel, without the reverse flowrelief valve 213 no water would flow due to the pressure required topush water back through the pump head 230. Therefore, the reverse flowrelief valve 213 is configured such that in normal operation it allowswater to pass through it from the pump to the fuel cell stack 110. Whensubjected to a small back pressure (for example in the region of 300mBar·g) when the pump head 230 is not being operated, a diaphragm opensand allows water to flow back into the flask through the purge port 238.

An exemplary embodiment of the reverse flow relief valve 213 is shown inFIG. 3 in cutaway form. In normal operation, water flows from thecontainment vessel 140 through the reverse flow relief valve 213 in thedirections indicated by arrows 301. Water flows through a first feedport 314, through a non-return valve 316 and out of the valve 213through a second feed port 320 towards the cathode water injection line125. The pressure of water in the first feed port 314 is transmitted viaa connecting passage 313 and transfer passage 312 to a cavity 311 sealedby a sealing valve, for example in the form of a diaphragm 321, andenclosed by a cover face 323. The pressure maintains a sealing face 317of the diaphragm 321 against a face of a bypass passage 318, and thusprevents fluid from passing between the second feed port 320 and a lowpressure cavity 315 behind the diaphragm 321.

Once the water containment vessel pump 230 is disabled, a loss ofpressure in the first feed port 314 and an increase in pressure in thesecond feed port 320 caused by an increased pressure in the cathodevolume of the fuel cell stack 110 causes the non-return valve 316 toclose. The increased pressure in the second feed port causes thediaphragm 321, which is preferably composed of a resilient material suchas a rubber, to flex and open up a passage between the bypass passage318 and the low pressure cavity 315. Water is then allowed to flow fromthe second feed port 320 through the bypass passage 318, into the lowpressure cavity 315, through a purge passage 322 and out of the valve213 through the purge port 238. The overall direction of flow in thereverse direction is indicated by arrows 302.

The configuration of the reverse flow relief valve allows water to beejected from the cathode volume in the fuel cell stack and the cathodewater injection line while allowing the pump head 230 in the containmentvessel 140 to remain primed with water. Provided the water 212 in thevessel is not frozen, the pump head 230 then remains in a state ready toimmediately begin pumping water for injection into the cathode volume ofthe fuel cell stack 110.

FIGS. 4 a and 4 b show schematically the two different modes ofoperation possible with the reverse flow relief valve 213. In FIG. 4 a,the purge operation is shown, in which low pressure air from the fuelcell stack enters the valve 213 through the second feed port 320, andthere is no flow into the first feed port 314. This low pressure aircauses the diaphragm 321 to deflect and allow flow through the bypasspassage 318 and out of the valve through the purge port 238 for deliveryto the water reservoir in the water containment vessel 140. Thenon-return valve 316 prevents flow through to the first feed port 314.

In FIG. 4 b, the water delivery operation of the reverse flow reliefvalve 213 is shown, in which high pressure water pumped from thereservoir in the water containment vessel 140 enters the valve 213through the first feed port 314. The pressure, transmitted via theconnecting passage 313 to the high pressure cavity 311, maintains thediaphragm 321 in a closed position against the bypass passage 318. Flowof water passes through the non-return valve 316 and out of the reverseflow relief valve 213 via the second feed port 320. No flow occursthrough the purge port 238.

Two alternative arrangements of the cathode exit, exhaust and waterejection lines are shown schematically in FIGS. 5 a and 5 b. In FIG. 5a, the water separator 131 is connected first in line with the cathodeexit line 121, in series with and before the heat exchanger 130, and twowater ejection lines 128 a, 128 b are used to send water via pumps 132a, 132 b to the containment vessel 140. In FIG. 5 b, the water separator131 is connected in series with, but after, the heat exchanger 130, anda single pump 132 is used to pump water through the water return line128 to the containment vessel 140.

In the configuration of FIG. 5 a, the cathode exit stream is passedthrough the cathode exit line 121 to a cyclonic water separator 131,which removes the liquid content before redirecting saturated air to aheat exchanger 130. The heat exchanger 130 cools the saturated airstream, which results in a proportion of the entrained water changing toliquid phase. Two pumps 132 a, 132 b are used to transfer recoveredwater to the containment vessel 140, one pump 132 a connected via waterreturn line 128 a to the base of the separator 131 and the other 132 bconnected via water return line 128 b to the heat exchanger 130 exitmanifold box. During normal fuel cell operation the variable shut offvalve 120 is held open. However, during fuel cell system shutdown, thisvalve is closed to back pressure the cathode flow path. In this case thecompressor 133 (or other device which supplies air to the fuel cellaccording to a defined flow rate set point) works harder to maintain afixed air flow rate. The separator 131 contains a pressure relief valve(not shown) which, during this back pressured shutdown phase, opensallowing ejection of water from the water separator 131 through a waterpurge line 510 (typically to atmosphere). Prior to back pressuring thecathode flow path at system shutdown, the pump 132 a is typicallyallowed to run for a few seconds to clear most of the water from theseparator 131.

In the configuration of FIG. 5 b, operation is similar to that of FIG. 5a, except that the cyclonic separator 131 is positioned after the heatexchanger 130. As such, the liquid element of the cathode exit stream ispassed through the heat exchanger 130. In addition, only one pump 132 isrequired to transfer the recovered water back to the containment vessel140. One advantage of this configuration is that as the inlet to theheat exchanger 130 is part way from the bottom, any residual water thatremains in the heat exchanger 130 after shutdown will fall to occupy thelower part of the heat exchanger 130. In the event of the heat exchanger130 then being subjected to sub-zero temperature, this water willfreeze. However, flow will still be possible through the cathode flowpath through the rest of the heat exchanger, which will then heat up anddefrost the frozen water in the lower part.

The purge port 510 in each case allows the cyclonic separator to becleared dry, allowing subsequent storage at low temperatures.

Preferably, the thermal inertia of the cyclonic separator 131 is lowsuch that when a small amount of liquid water enters the separator whenthe separator 131 is below 0° C., the liquid water does not freeze.

Optionally, a pressure relief valve may be used to increase the backpressure of the system to such an extent that the transfer pumps 132,132 a, 132 b are not required. However, in this instance the cyclonicrelief valve and purge port 510 might not be fitted, to ensure that theinternal pressure in the cathode water exit line 121 forces waterthrough the water ejection line towards the water containment vessel140. If no transfer pump 132 is fitted, the arrangement shown in FIG. 5b is more preferable, to ensure that water ejected from the cathode exitline passes through a single water ejection line 128.

During operation of the fuel cell system 100 (with reference to FIGS. 1and 2), water from the containment vessel 140 is pumped through thecathode flow path via the cathode water injection line 125 and thecathode water injection inlet 127. After passing through the cathodevolume within the fuel cell stack 110, water passes out of the stack 110via the cathode water exit line 121 and into the heat exchanger 130. Amixture of exhaust gas and condensed water passes through the waterseparator 131. Condensed water then passes through the water return line128 and into the water containment vessel 140. Any excess water isejected through the water overflow line 144. Exhaust gases are ejectedthrough the exhaust shut-off valve 120, which is held at least partiallyopen to control the pressure within the cathode flow path.

With reference to FIG. 1, fuel gas is fed into the anode inlet 156 andinto the anode volume (not shown) within the fuel cell stack 110. Valves153, 161 are operated to maintain a desired pressure within the anodevolume. The manual valve 162 connected to the anode outlet 159 remainsclosed. Optionally, water ejected from the anode exhaust stream isseparated into liquid and gas phases with a further water separator 163.

On shutdown of the fuel cell system 100, the fuel supply to the fuelcell stack 110 is first shut off by closing the solenoid actuated valve153 on the fuel supply line 155. The shut-off valve 120 on the cathodeexhaust line 122 is then closed, while the air compressor 133 continuesoperation. In practice a time may be required to flush through thecathode flow path with air before the shut-off valve 120 is closed. Thepressure in the cathode flow path then rises. The water ejection linepump 132, if present, preferably continues operation for a time afterthe shut-off valve is closed, to allow water to continue passing throughto the containment vessel 140. The containment vessel pump 230 ceasesoperation, and water consequently stops being fed into the cathode waterinjection line 125.

A short purging operation of the anode flow path may also be used duringshutdown to eject water present in the anode flow path, with water beingforced through opened anode exit valve 161, followed by depressurisationof the anode volume in the fuel cell stack 110.

Air being flushed through the cathode flow path forces residual waterout of the cathode volume in the fuel cell stack 110 and through thecathode exit line 121, through the heat exchanger 130 to the waterseparator 131. The pump 132 pumps water from the separator 131 throughthe water return line 128 and into the water containment vessel 140.When the shut-off valve 120 closes the cathode air pressure will rise,forcing water out of the cathode volume in the fuel cell stack 110through the cathode water injection inlet 127 and through the cathodewater injection line 125 and the purge port 238 of the reverse flowrelief valve 213 towards the water containment vessel 140.

The ability to purge water from the cathode flow path through to thewater containment vessel 140 allows water, which may be trapped ininternal features and water distribution galleries, to be removed. For atypical size of fuel cell stack for automotive applications, this mayresult in around 30 ml of water being removed from the cathode flowpath. The use of the air compressor 133 rather than a nitrogen purgefeed reduces the number of components required and avoids excessivedrying out of the fuel cell stack. The membranes in the stack can thenbe kept in a more suitable state for a subsequent start-up operation.Timed operation of the air compressor on shutdown can be optimized toprovide a balance between removing sufficient water to prevent adverseeffects from sub-zero conditions and dehydration of the membranes. For atypical fuel cell system, the air compressor may be operated around 1 to2 minutes after closing the exhaust valve 120. Hydrogen gas may also beused to purge excess water from the anode flow path.

While the method and agent have been described in terms of what arepresently considered to be the most practical and preferredimplementations, it is to be understood that the disclosure need not belimited to the disclosed embodiments. It is intended to cover variousmodifications and similar arrangements included within the spirit andscope of the claims, the scope of which should be accorded the broadestinterpretation so as to encompass all such modifications and similarstructures. The present disclosure includes an and all implementationsof the following claims.

It should also be understood that a variety of changes may be madewithout departing from the essence of the disclosure. Such changes arealso implicitly included in the description. They still fall within thescope of this disclosure. It should be understood that this disclosureis intended to yield a patent covering numerous aspects bothindependently and as an overall system and in both method and apparatusmodes.

Further, each of the various elements of the disclosure and claims mayalso be achieved in a variety of manners. This disclosure should beunderstood to encompass each such variation, be it a variation of animplementation of any apparatus implementation, a method or processimplementation, or even merely a variation of any element of these.

Particularly, it should be understood that as the disclosure relates toelements of the implementation, the words for each element may beexpressed by equivalent apparatus terms or method terms—even if only thefunction or result is the same.

Such equivalent, broader, or even more generic terms should beconsidered to be encompassed in the description of each element oraction. Such terms can be substituted where desired to make explicit theimplicitly broad coverage to which this disclosure is entitled.

It should be understood that all actions may be expressed as a means fortaking that action or as an element which causes that action.

Similarly, each physical element disclosed should be understood toencompass a disclosure of the action which that physical elementfacilitates.

Any patents, publications, or other references mentioned in thisapplication for patent are hereby incorporated by reference. Inaddition, as to each term used it should be understood that unless itsutilization in this application is inconsistent with suchinterpretation, common dictionary definitions should be understood asincorporated for each term and all definitions, alternative terms, andsynonyms such as contained in at least one of a standard technicaldictionary recognized by artisans and the Random House Webster'sUnabridged Dictionary, latest edition are hereby incorporated byreference.

Finally, all referenced listed in the Information Disclosure Statementor other information statement filed with the application are herebyappended and hereby incorporated by reference; however, as to each ofthe above, to the extent that such information or statementsincorporated by reference might be considered inconsistent with thepatenting, such statements are expressly not to be considered as made bythe applicant(s).

In this regard it should be understood that for practical reasons and soas to avoid adding potentially hundreds of claims, the applicant haspresented claims with initial dependencies only.

Support should be understood to exist to the degree required under newmatter laws—including but not limited to United States Patent Law 35 USC132 or other such laws—to permit the addition of any of the variousdependencies or other elements presented under one independent claim orconcept as dependencies or elements under any other independent claim orconcept.

To the extent that insubstantial substitutes are made, to the extentthat the applicant did not in fact draft any claim so as to literallyencompass any particular embodiment, and to the extent otherwiseapplicable, the applicant should not be understood to have in any wayintended to or actually relinquished such coverage as the applicantsimply may not have been able to anticipate all eventualities; oneskilled in the art, should not be reasonably expected to have drafted aclaim that would have literally encompassed such alternatives.

Further, the use of the transitional phrase “comprising” is used tomaintain the “open-end” claims herein according to traditional claiminterpretation. Thus, unless the context requires otherwise, it shouldbe understood that the term “compromise” or variations such as“comprises” or “comprising”, are intended to imply the inclusion of astated element or step or group of elements or steps but not theexclusion of any other element or step or group of elements or steps.

Such terms should be interpreted in their most expansive forms so as toafford the applicant the broadest coverage legally permissible.

All callouts associated with figures are hereby incorporated by thisreference.

Since certain changes may be made in the above apparatus withoutdeparting from the scope of the invention herein involved, it isintended that all matter contained in the above description, as shown inthe accompanying drawing, shall be interpreted in an illustrative, andnot a limiting sense.

The invention claimed is:
 1. A fuel cell system comprising a reverseflow relief valve comprising: a first feed port; a second feed port; anon-return valve within a main fluid passage extending between the firstand second feed ports, the non-return valve configured to allow fluid topass from the first to the second feed ports and to block passage offluid in the reverse direction; a bypass fluid passage in fluidcommunication with the main fluid passage; and a sealing valve biasedagainst an end of the bypass passage between the bypass passage and apurge port, wherein the sealing valve is configured to maintain a sealagainst the bypass passage when fluid pressure in the first feed portexceeds fluid pressure at the second feed port to prevent fluid flowfrom the main fluid passage to the purge port through the bypass fluidpassage, and to allow fluid flow from the second feed port to the purgeport through the bypass fluid passage when fluid pressure at the secondfeed port exceeds fluid pressure at the first feed port, wherein thesecond feed port of the reverse flow relief valve is connected to acathode water injection line, the first feed port is connected to a pumpand the purge port is in fluid communication with an interior volume ofa water containment vessel.
 2. The reverse flow relief valve of claim 1wherein the sealing valve is configured to allow fluid flow from thesecond feed port to the purge port through the bypass fluid passage whenfluid pressure at the second feed port exceeds fluid pressure at thefirst feed port by a predetermined amount.
 3. The reverse flow reliefvalve of claim 2 wherein the predetermined amount is no less than 300mbar.
 4. The reverse flow relief valve of claim 1 wherein an opposingface of the diaphragm is in fluid communication with the first feedport.
 5. The reverse flow relief valve of claim 1 comprising aconnecting fluid passage extending between the first feed port and thesealing valve.